forked from OSchip/llvm-project
d1522513d4
To set up a tail-predicated loop, we need to to calculate the number of elements processed by the loop. We can now use intrinsic @llvm.get.active.lane.mask() to do this, which is emitted by the vectoriser in D79100. This intrinsic generates a predicate for the masked loads/stores, and consumes the Backedge Taken Count (BTC) as its second argument. We can now use that to reconstruct the loop tripcount, instead of the IR pattern match approach we were using before. Many thanks to Eli Friedman and Sam Parker for all their help with this work. This also adds overflow checks for the different, new expressions that we create: the loop tripcount, and the sub expression that calculates the remaining elements to be processed. For the latter, SCEV is not able to calculate precise enough bounds, so we work around that at the moment, but is not entirely correct yet, it's conservative. The overflow checks can be overruled with a force flag, which is thus potentially unsafe (but not really because the vectoriser is the only place where this intrinsic is emitted at the moment). It's also good to mention that the tail-predication pass is not yet enabled by default. We will follow up to see if we can implement these overflow checks better, either by a change in SCEV or we may want revise the definition of llvm.get.active.lane.mask. Differential Revision: https://reviews.llvm.org/D79175 |
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README.txt
The LLVM Compiler Infrastructure ================================ This directory and its subdirectories contain source code for LLVM, a toolkit for the construction of highly optimized compilers, optimizers, and runtime environments. LLVM is open source software. You may freely distribute it under the terms of the license agreement found in LICENSE.txt. Please see the documentation provided in docs/ for further assistance with LLVM, and in particular docs/GettingStarted.rst for getting started with LLVM and docs/README.txt for an overview of LLVM's documentation setup. If you are writing a package for LLVM, see docs/Packaging.rst for our suggestions.